Mast cells have long been known to be important cellular causes of asthma and allergy. Recent studies have challenged that pathophysiological paradigm in favor of a much more diverse role for mast cells in various aspects of immunity. Conflicting reports in the literature have assigned both effector and suppressor roles to mast cells based on how they are stimulated. While it is clear that mast cells are potent innate immune cells essential for early clearance of pathogens, such as bacteria and parasites, far less is known about what role mast cells play in adaptive immunity, especially with respect to their effect on helper T cell immunity. Our preliminary experiments using major histocompatibility complex class II (MHCII) tetramers to examine mast cell effects on endogenous, antigen-specific helper CD4 T cell immunity showed that inducing mast cell degranulation in the tissue led to a significant increase in antigen-specific helper CD4 T cell numbers in draining lymph nodes. Further, we established that a lack of mast cells led to a marked decrease in antigen-specific CD4 T cell accumulation in antigen-containing tissue. These combined results, in addition to our past work, lead us to the hypothesis that mast cells contribute to endogenous helper T cell population expansion and migration and, ultimately, regulate helper T cell functional phenotypes. We further posit that this T cell activation is mediated through direct mast cell effects on CD4 T cells, including cytokine production by mast cells and that this is determined by how mast cells are initially activated. We will test this hypothesis using MHCII tetramers to analyze endogenous, antigen-specific CD4 T cells combined with mast cell deficient mice or mast cell deficient mice reconstituted with bone marrow derived mast cells. Aim 1 will examine the role for mast cells in initiating and maintaining the antigen-specific CD4 T cell response in antigen-containing tissue and antigen-draining lymph nodes while Aim 2 will assess the function of mast cells in driving CD4 T cell Th phenotypes in response to different mast cell activation stimuli. These studies are designed to examine multiple mast cell effects on antigen-specific CD4 helper T cell immunity. This investigation should provide novel opportunities to guide the response in favor of pathogen elimination leading to better mast cell based therapeutic interventions benefitting human health.